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Unit 51: Electrical Technology The Characteristics and Principles of AC and DC Generators and the features of a Range of difference Power Station Course Aims • NDGTA At the end of this course the learner will be able to… 1. Know the methods used to produce electrical energy 2. Know the properties and applications of conductors, insulators and magnetic materials 3. Know the physical arrangements of supply, transmission and distribution equipment 4. Know how electrical energy is used to support applications of electrical technology Agenda NDGTA • At the end of the session the learner will be able to… – Describe DC Power systems D.C. Power Systems NDGTA • D.C. Power systems have a multitude of applications…. – Use in electroplating industry – Use in cars – Portable equipment • Most electrical generation produced in the UK is 3-phase A.C. which is converted to D.C. for use. D.C. Power Systems NDGTA • D.C. is also available in the form of primary and secondary chemical cells. • D.C. generators also produce electrical power for specialist applications. D.C. Production using Chemical Cells NDGTA • An electrochemical cell is composed of two dissimilar metals, which are immersed in a conductive liquid or paste called an electrolyte. • Chemical cells are classified as either primary or secondary cells: primary cells are ordinarily not usable after a certain time period i.e. the chemicals are normally used up and cannot produce any further electrical energy; secondary cells can be renewed after they are used by reactivating (i.e. charging) the chemical process. • When two or more cells are connected in series they form a battery Characteristics of a Primary Cell NDGTA Characteristics of a Primary Cell NDGTA • When the chemicals that compose the cell are brought together, their molecular structure is altered. • During this alteration atoms may gain additional electrons or lose some of their electrons. • This ionisation process develops a chemical solution capable of conducting an electrical current. • The voltage of a primary cell depends upon the electrode material and the type of electrolyte used. (For a zinc-carbon dry primary cell this is typically about 1.5 v) Internal Resistance NDGTA • An important characteristic of a chemical cell is its internal resistance • V = I(R + r) • R is the resistance of the load and r in the internal resistance of the cell both measured in Ohms. Internal Resistance I V R r NDGTA Internal Resistance NDGTA • Given a 1.5 v battery has an internal resistance of 0.8 Ω find the current when a load of 10.0 Ω is connected to the battery? • The no-load voltage of a battery is 9.05 v. Its rated load is 200 mA and its internal resistance is 0.1 Ω. Find the rated output voltage of the battery? • The rated output of a battery is 30 v and its noload voltage is 30.15 v. Its FLC is 350 mA. Find the internal resistance of the battery? Primary Cells NDGTA • Explore the applications of primary cells Characteristics of a Secondary Cell NDGTA • Chemicals of a secondary cell may be reactivated by a charging process. • Secondary cells are sometimes called storage cells • The most common types of secondary cells are lead-acid cell, nickel-iron (Edison) cell and the nickel-cadmium cell. The Lead-Acid Cell NDGTA • The electrodes of the lead-acid cell are made of lead (Pb is the negative plate) and lead-peroxide (PbO2 is the positive plate). The electrolyte is dilute sulphuric acid (H2SO4). • When a lead-acid cell supplies current to a load connected to it the chemical process can be expressed as… • PbO2 + Pb + 2H2SO4 -> 2PbSO4 + 2H2O The Lead-Acid Cell NDGTA • The sulphuric acid ionises to produce 4-positive hydrogen ions (H+) and 2-negative sulphate (SO4-) ions. • A negative charge is developed on the lead plate when the SO4- ion combines with the lead plate to form lead sulphate (PbSO4). • The positive hydrogen ions (H+) combine with electrons of the lead-peroxide plate and become neutral hydrogen atoms. They next combine with oxygen (O) on this plate to become water (H2O). The lead-peroxide plate thus becomes positively charged. • A fully charged lead-acid cell has an electrical potential developed between the electrodes of around 2.0 volts. The Lead-Acid Cell NDGTA • After discharging by supplying a current to a load for a certain period of time, it no longer is able to develop an output voltage. • The cell may be recharged by causing direct current to flow through the cell in the opposite direction • 2PbSO4 + 2H2O -> PbO2 + Pb + 2H2SO4 Other Secondary Cells NDGTA • Explore the construction of the nickel-iron and the nickel-cadmium cells. • Explore the applications of secondary cells A Battery NDGTA • A battery is a combination of more than one cell. • The cells in a battery may be connected in series or in parallel. – In series: • The total emf = sum of each of the cell’s emfs • The total internal resistance = sum of each of the cell’s internal resistances – In parallel: • If each cell has the same emf and internal resistance, then.. • The total emf = emf of one cell • The total internal resistance of n cells = 1/n = internal resistance of one cell Problems NDGTA • Eight cells, each with an internal resistance of 0.2 Ω and an emf of 2.2 v are connected (a) in series and (b) in parallel. Determine the emf and the internal resistance of the batteries? • A cell has an internal resistance of 0.02 Ω and an emf of 2.0 v. Calculate its terminal p.d. if it delivers (a) 5 A (b) 50 A • The pd at the terminals of a battery is 25 v when no load in connected and 24 v when a lad taking 10 A is connected. Determine the internal resistance of the battery. • Ten 1.5 v cells, each having an internal resistance of 0.2 Ω are connected in series to a load of 58 Ω. Determine the current flowing in the circuit and (b) the pd at the battery terminals Further information on Batteries NDGTA • As cells can be placed in series within one another to form a battery; so to can batteries be placed in series with one another. Note: the 1.5 V 1.5 V 3 internal resistance of the series of batteries is the sum of the internal resistance of each of the batteries NDGTA Batteries in Series • Consider the following arrangement of batteries… 1Ω 2Ω 1Ω Q P 4V 5V What would be the total emf and internal resistance across P – Q? 3V Batteries in Parallel NDGTA • Consider the following arrangement of batteries… 2Ω P 1.5 V 2Ω Q 1.5 V What would be the total emf and internal resistance across P – Q? Safe disposal of batteries NDGTA • Battery disposal has become a topical subject in the UK because of greater awareness of the dangers and implications of depositing up to 300 million batteries per annum – a waste stream of over 20,000 tonnes – into landfill sites. • Certain batteries contain substances which can be a hazard to humans, wildlife and the environment, as well as posing a fire hazard. • Other batteries can be recycled for their metal content Safe disposal of batteries NDGTA • Waste batteries are a concentrated source of toxic heavy metals such as mercury, lead and cadmium. • If batteries containing heavy metals are disposed of incorrectly, the metals can leach out and pollute the soil and groundwater, endangering humans and wildlife. – Long-term exposure to cadmium, a known carcinogen (i.e. a substance producing cancerous growth), can cause liver and lung disease. – Mercury can cause damage to the human brain, spinal system, kidneys and liver. – Sulphuric acid in lead acid batteries can cause severe skin burns or irritation upon contact. • It is increasingly important to correctly dispose of all types of batteries. Safe disposal of batteries NDGTA • Refer to Handout 1 and note the disposal recycling options • Battery disposal has become more regulated since the Landfill Regulations 2002 and Hazardous Waste Regulations 2005. • From the Waste Electrical and Electronic Equipment (WEEE) Regulations 2006, commencing July 2007 all producers (manufacturers and importers) of electrical and electronic equipment will be responsible for the cost of collection, treatment and recycling of obligued WEEE generated in the UK. Alternative and Renewable Energy Sources NDGTA • Alternative energy sources are sources of energy that could replace the coal, oil or gas – all of which release carbon when burned. • Renewable energy implies that it derives from a source which is automatically replenished (or one which is effectively infinite) so that it is not depleted when used • Coal, oil and gas are not renewable – their supplies will eventually run out.